A common approach for increasing throughput for mobile terminals in a cellular network is to deploy more and more base stations, thereby increasing the available bandwidth. In other words, in geographical areas with many mobile terminals, increasing the base station density would facilitate serving all mobile terminals with a high data rate in a given area. Although this approach could theoretically provide significant rate improvements, it has two main problems; 1) it is very expensive, both in cost and in time, and 2) today's cities are already saturated with base stations and people are more and more reluctant to tolerate further base stations in the cities because of electromagnetic emission.
The most promising and practical approach, which provides a fast deployable and cost efficient solution, is heterogeneous networks (HetNets). The concept is very simple; the idea is to deploy smaller base stations in areas where the data requirements are the highest in order to provide a good service even in crowded geographical areas, where it is not possible to deploy more standard base stations. These smaller base stations are much cheaper and transmit with a significantly lower power. It enables a commercially viable deployment as well as taking health concerns into account.
There exist several types of base stations in HetNets.                Macro nodes: these are the standard base stations, as deployed nowadays. They consume the most energy, transmit with the highest power and can therefore serve mobile terminals at the furthest distance. They typically use a transmit power in the order of 40 W. They are designed to cover larger areas like city districts. The area served by a macro node is called a macro cell.        Pico nodes: these are smaller base stations, which are cheaper than macro nodes and have a significantly lower transmit power. They can serve UEs in much smaller geographical areas, e.g., a mall or a metro station, and are commonly referred to as a hotspot. Pico nodes typically use a transmit power in the order of 1 W. The area served by a pico node is called a pico cell.        Femto nodes: these are the smallest base stations. They are typically used to cover a small office or a house.        
In current mobile communication networks, a mobile terminal is served by a macro node. If this mobile terminal moves too far from the macro node, it proceeds to a handover and simply changes its serving macro node. In a network with only macro nodes, the macro nodes are typically placed such that mobile terminals close to the centre of the macro cell experience little interference. Such communication networks are not very sensitive to poor cell selection schemes.
In HetNets on the contrary, the cell selection of mobile terminals plays a crucial role and is a fundamental problem to be solved for providing a successful HetNet deployment. Indeed, pico nodes are typically placed inside a macro cell to improve the data rate at specific locations. Since the macro node has a much higher transmit power than the pico node, the mobile terminals served by the pico node suffer a very large interference from the macro node. Associating the right mobile terminals to the right node is therefore a problem that cannot be treated as in standard mobile communication networks only relying on macro nodes.
When a mobile terminal wants to join the cellular network, it first has to find cells in its neighbourhood and then select which one of them it will be associated to. Cell-specific reference signals are sent periodically by the base stations and are used by the mobile terminals to estimate their channel quality, i.e., the power received from the base station. These reference signals are known in advance at the mobile terminal and can be used, for example to calculate the reference signal received power (RSRP), which is basically the average received power of the reference signal transmitted by the base station to the mobile terminal per transmitted resource element. The role of the cell selection algorithm is to decide which cell to connect to, based on such measurements performed on the cells within range for the mobile terminal.
Typically, cell selection occurs periodically, e.g., when channel conditions have changed, and also based on network churn, i.e., when a mobile terminal enters or leaves a cell.
There exist two main approaches to this problem that are currently being implemented in Long Term Evolution (LTE) Advanced networks.
The RSRP approach is the simplest method for associating mobile terminals to base stations. At the time a mobile terminal needs to be associated to a base station, it measures the received power from each of its neighbouring base stations. The mobile terminal is then associated to the base station with the largest received power. This algorithm has been used in Universal Mobile Telecommunications System (UMTS) and is still used in LTE. Its strength is its conceptual simplicity as well as its profoundly decentralized nature. Indeed, it only requires for the mobile terminals to measure the received power values and to report the largest to the network. For macro-only networks, this method has proven very efficient and is the corner stone of today's cell selection algorithms.
In HetNets however, the RSRP method suffers from the transmit power asymmetry between nodes. Since a macro node has a much larger transmit power than a pico node, most mobile terminals will experience a larger received power from the macro node. This leads to a strong load imbalance between the macro nodes and the pico nodes, leaving the pico nodes underutilized. To make the best use of the increase in available bandwidth provided by the pico node, a more balanced distribution of mobile terminals is desired. A pico node will not live up to its full potential in densely populated areas if most of the users are still associated with macro nodes.
To tackle the main problem of the RSRP method, while preserving its simplicity, the so called Cell Range Extension (CRE) method was introduced. Considering a best Signal-to-Noise-Ratio (SNR) heuristic, all mobile terminals which receive their largest received power from a certain node can be said to be in its range. If the most remotely located mobile terminal that is still in the range of this node is identified, the distance of the mobile terminal from the base station is interpreted as a radius, and a circular area is imagined with this radius and the base station as a centre, a so-called range area of this base station is attained, and any mobile terminal within the range area will be associated to the base station.
Clearly, in a HetNet, the range of a pico node is rather small, since its transmit power is small compared to the one of neighbouring macro nodes. The main idea of the CRE method is to virtually increase the range of the pico nodes by a fixed factor. The difference with the RSRP method is that a mobile terminal only is associated to a macro node if its received power is better than the one of a pico node multiplied by the fixed extension factor.
The CRE method keeps the major advantage of simplicity from the RSRP method while enabling to balance the distribution of the users between the macro and pico nodes. This way the pico node is guaranteed to be serving a significant amount of mobile terminals. A drawback of this algorithm is that mobile terminals are not necessarily associated to the node with the largest received power possible. Consequently, a mobile terminal might experience a very poor SINR and therefore be incapable of receiving any data. Further, mobile terminals at the border of the macro and pico cells will receive a very large interference from the macro node, leading to a very poor throughput.
In general, the cell selection is performed more seldom than the resource allocation. In other words, the channel condition can vary a lot between occasions when mobile terminals are re-associated with base stations. Further, a mobile terminal might have a long data stream to transmit; so long that new mobile terminals enter and exit the network during this transmission. This could lead to completely different resource sharing distribution.
Existing cell selection schemes do not take into account changes in the channel and network conditions and only use knowledge pertaining to instantaneous physical properties, like for instance signal strength of the base station as received at the mobile terminals.